The present invention relates to a catalyst for purifying the exhaust gases of diesel engines, which catalyst contains at least one zeolite and, additionally, at least one support oxide selected from the group consisting of aluminum oxide, silicon dioxide, titanium dioxide and aluminum silicate and at least one noble metal selected from the group consisting of platinum, palladium, rhodium, iridium, gold and silver.
The exhaust gas of diesel engines contains carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOx), sulfur dioxide (SO2) and carbon black particles as atmospheric pollutants. The unburned hydrocarbons include paraffins, olefins, aldehydes and aromatic compounds. In comparison with the exhaust gases of gasoline engines, diesel exhaust gases contain a substantially higher proportion of not readily oxidizable, long-chained paraffins, which are for a large part condensed onto the carbon black particles as so-called VOF""s (VOF=volatile organic fraction) and hence increase the particulate load of the exhaust gas. The sulfur dioxide contained in the exhaust gas originates from the sulfur content of the diesel fuel. By oxidation to sulfur trioxide, sulfates may form which likewise accumulate on the carbon black particles and increase the mass of the particles.
Moreover, diesel exhaust gases are leaner than stoichiometric in composition, which means that their oxygen content is higher than would be necessary for complete combustion of all combustible constituents of the exhaust gas. The oxygen concentration in diesel exhaust gas is usually from 3 to 10 vol. %, whereas in stoichiometrically composed exhaust gases of gasoline engines it is only about 0.7 vol. %.
The high oxygen concentration of diesel exhaust gases is based on the fact that diesel engines are operated with a high air/fuel ratio (kg of air/kg of fuel) of over 18. By contrast, stoichiometrically operated gasoline engines work with an air/fuel ratio of 14.6, which permits stoichiometric combustion of the hydrocarbons.
A further peculiarity of diesel exhaust gases is their substantially lower temperature as compared with the temperature of gasoline engines. In part-load operation, the exhaust gas temperature of modern diesel engines is in the range from 120 to 250xc2x0 C., and it reaches a maximum temperature of from 550 to 650xc2x0 C. only in full-load operation.
The actual composition of the exhaust gas of a diesel engine depends on the type of engine in question. Moreover, the development of diesel engines in the last 15 years has led to a continual change in the composition of diesel exhaust gas. Important developmental steps in this connection were the introduction of exhaust gas recirculation (EGR) and the continual further development of fuel injection systems such as xe2x80x9cunit injectorxe2x80x9d and xe2x80x9ccommon-railxe2x80x9d. As a result of such developments it has been possible to reduce still further the nitrogen oxide emissions of diesel engines, which were already low as compared with gasoline engines, and exhaust gas temperatures have constantly been lowered further. Modern diesel engines for motor-cars exhibit nitrogen oxide emissions of less than 100 vol.-ppm in the majority of operating states of the engine.
The described particularities of diesel exhaust gases have made it necessary to introduce special exhaust gas purification systems for diesel engines. Successes in engine development and ever more stringent legal requirements as regards permissible emissions demand the continual further development of existing exhaust gas purification systems for diesel engines.
As well as reducing the particulate emission of diesel engines by introducing suitable diesel soot filters, the fundamental question initially was to reduce emissions of hydrocarbons by introducing suitable oxidation catalysts. Since the nitrogen oxide emissions of diesel engines were markedly higher about 10 years ago than they are today, it was important when developing such oxidation catalysts also to inhibit the further oxidation of nitrogen monoxide contained in the exhaust gas to nitrogen dioxide and the further oxidation of sulfur dioxide to sulfur trioxide.
Diesel oxidation catalysts having a reduced tendency to oxidize nitrogen monoxide and sulfur dioxide are described, for example, in patent specifications DE 39 40 758 C2, DE 42 13 018 C1 and U.S. Pat. No. 5,911,961.
A further step was the development of so-called xe2x80x9clean-NOxxe2x80x9d catalysts. Such catalysts are capable of reducing nitrogen oxides even in oxygen-rich exhaust gases. The unburned hydrocarbons still present in the diesel exhaust gas serve as the reducing agent. If the content of such hydrocarbons in the exhaust gas is not sufficient for the reduction of the nitrogen oxides, it can be increased accordingly by means of suitable measures in engine control or by the separate injection of diesel fuel. Of course, that also leads to a higher fuel consumption.
Lean-NOx catalysts are described in DE 196 14 540 A1, in EP 0 427 970 A2, in EP 0 920 913 A1 and in U.S. Pat. No. 5,897,846.
There have also become known catalysts that are said to improve the conversion of hydrocarbons and also of nitrogen oxides by storing the hydrocarbons at low exhaust gas temperatures and releasing them at higher exhaust gas temperatures. Such catalysts are described in U.S. Pat. No. 5,849,255, in WO 94/22564 and in WO 96/39244.
In the case of the first-mentioned oxidation catalysts, the reduced tendency to oxidize nitrogen monoxide and sulfur dioxide is achieved by additions of tungsten, antimony, molybdenum, nickel, vanadium, manganese and others. Vanadium is preferably used. Accordingly, the active component of the catalyst according to DE 39 40 758 C2 consists of platinum, palladium, rhodium and/or iridium in contact with vanadium or with an oxidic vanadium compound. The active component is deposited on finely divided aluminum oxide, titanium dioxide, silicon dioxide, zeolite and mixtures thereof. In order to prepare the catalyst, the oxidic support materials are first applied in the form of a dispersion coating to an inert carrier body. The dispersion coating is then impregnated with the active components. If mixtures of the various support oxides are used for the dispersion coatings, then all the constituents of the dispersion coating are coated uniformly with the active components in the subsequent impregnation.
DE 42 13 018 C1 also describes the use of aluminum oxide, titanium dioxide, silicon dioxide and zeolite as supports for the catalytically active components, which are present in the form of noble metals platinum, palladium, rhodium and/or iridium doped with vanadium or in contact with an oxidic vanadium compound.
The oxidation catalyst according to U.S. Pat. No. 5,911,961 contains on a first support oxide platinum and/or palladium in conjunction with at least one metal selected from the group tungsten, antimony, molybdenum, nickel, manganese, iron, bismuth and others. The catalyst additionally contains further oxides selected from the group aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide, aluminum silicate and, inter alia, also zeolites. The additional oxides are not coated with the catalytically active components.
The lean-NOx catalyst according to DE 196 14 540 A1 contains one or more zeolites and at least one platinum group metal. The catalyst additionally contains one or more support oxides selected from the group aluminum silicate, aluminum oxide and titanium dioxide. The catalytically active noble metals of that catalyst are deposited only on the additional support oxides.
EP 0 427 970 A2 describes a lean-NOx catalyst for decreasing the nitrogen oxides in an oxidizing exhaust gas having an air/fuel ratio of 22. The catalyst contains at least one zeolite having a molar ratio SiO2/Al2O3 of greater than 10 and pore diameters of from 0.5 to 1 nm. Noble metals are deposited on the zeolites; for each platinum group metal, the weight ratio of the metal to the zeolite must not fall below a minimum value if good rates of conversion of nitrogen oxides are to be maintained even after ageing of the catalyst.
EP 0 920 913 A1 describes a process for the preparation of a further lean-NOx catalyst. As in DE 196 14 540 A1, that catalyst contains a plurality of zeolites as well as further support oxides and catalytically active noble metals, it being ensured by the preparation that the zeolites do not come into contact with the catalytically active components. As a result, the zeolites are to be prevented from being coked by the hydrocarbons contained in the exhaust gas.
The lean-NOx catalyst according to U.S. Pat. No. 5,897,846 contains zeolites as support oxides for the catalytically active components. The catalytically active components are arranged on the zeolites at the outer surface in a shell having a thickness not exceeding 500 xc3x85.
U.S. Pat. No. 5,849,255 describes an oxidation catalyst in which noble metals selected from the platinum group are deposited on two different support material components. The smaller support material component consists of aluminum oxide, titanium dioxide and mixtures thereof, while the larger support material component consists of zeolites. The high content of zeolites in the catalyst is required to store the hydrocarbons contained in the exhaust gas at low exhaust gas temperatures and release them again at higher exhaust gas temperatures and supply them to the oxidation by the noble metals.
WO 94/22564 claims an oxidation catalyst for treating diesel exhaust gases, which catalyst contains cerium oxide, zeolites and optionally aluminum oxide. The catalyst may optionally also contain platinum. The oxidation catalyst oxidizes the hydrocarbons and carbon monoxide contained in the diesel exhaust gas, and the VOF""s. The hydrocarbons are retained in the pores of the zeolites during the cold-start phase or other phases during which the catalyst is relatively cold, until they can be effectively freed and oxidized by the catalyst during periods of relatively high temperature.
WO 96/39244 describes a catalyst for reducing the nitrogen oxide emissions of a diesel engine, which catalyst contains an adsorbent for hydrocarbons and a lean-NOx catalyst. Zeolites may be used as the adsorbent. The adsorbent adsorbs unburned hydrocarbons during colder phases of the operating cycle and releases them again during hotter phases of the operating cycle, so that the nitrogen oxides contained in the exhaust gas can be reduced thereby.
DE 197 53 738 A1 discloses a process for the preparation of an oxidation catalyst for treating diesel exhaust gases, which catalyst contains an aluminum silicate and a zeolite on which platinum in highly dispersed distribution is deposited. By suitably managing the impregnation of a powder mixture of aluminum silicate and zeolite, it is ensured that the platinum crystallites are deposited almost exclusively on the aluminum silicate.
An object of the present invention is to make available an improved oxidation catalyst for purifying the exhaust gases of modern diesel engines of motor-cars, the exhaust gases of which exhibit only low nitrogen oxide emissions, for example as a result of exhaust gas recirculation, and which contain only a small amount of sulfur dioxide owing to the use of low-sulfur fuels. The catalyst is in particular to exhibit a high oxidizing activity for carbon monoxide and hydrocarbons even at temperatures from 120 to 170xc2x0 C. and that oxidizing activity is to have high resistance to ageing.
The above and other objects can be achieved by the present invention by a catalyst for purifying the exhaust gases of diesel engines that contains at least one zeolite and, additionally, at least one support oxide selected from the group consisting of aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide and aluminum silicate and mixed oxides thereof and at least one of the noble metals selected from the group consisting of platinum, palladium, rhodium, iridium, gold and silver. The catalyst is characterized in that the atoms of the noble metals have a mean oxidation number of less than +2.5, on average more than 3 metal ligands and less than 3 oxygen ligands and are present on the zeolites and carrier oxides in the form of crystallites having a mean particle diameter size of from 1 to 6 nm.
Within the scope of this invention, a distinction is made between zeolites on the one hand and support oxides such as aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide and aluminum silicate and mixed oxides thereof on the other hand, even though zeolites may also be used as support material for catalytically active components. Within the scope of this invention, a distinction is made in particular between aluminum silicates and zeolites. Zeolites are aluminum silicates having a particular crystal structure, which has a pronounced pore structure. By contrast, the crystal structure of the support oxides designated aluminum silicates within the scope of this invention is similar at low silicon dioxide contents to the structure of the aluminum oxides and becomes amorphous as the concentration of silicon dioxide increases. The crystal structure of those aluminum silicates is therefore markedly different from the structure of the zeolites, even where the composition as represented by a combination of elements is the same.
Support oxides and zeolites act in the catalyst as support materials for the noble metals. In order to enable the noble metals to be deposited in as highly a dispersed manner as possible on the support materials, preference is given to support materials having a high specific surface area (BET surface area; determined by evaluation of nitrogen adsorption isotherms according to DIN 66132) of more than 5 m2/g.
The catalyst according to the invention exhibits especially low light-off temperatures for carbon monoxide and hydrocarbons. That is achieved by the combination of the support oxides aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide and aluminum silicate and mixed oxides thereof with one or more zeolites, and by a high degree of dispersion and a highly metal-like state of the crystallites of the noble metals deposited on those materials. Accordingly, both groups of materials, both the support oxides and the zeolites, serve in the catalyst according to the invention as support materials for the catalytically active noble metals. xe2x80x9cMetal-likexe2x80x9d as the term is used herein means that the metal atoms in the crystallites have a mean oxidation number of less than +2.5 and on average more than 3 metal ligands and less than 3 oxygen ligands.
It has been found that a high catalytic activity can be achieved when the crystallites of the noble metals have mean particle diameters of less than 6 nm and at the same time are metal-like in the sense described above. These are two conflicting requirements, which can be fulfilled simultaneously only by maintaining particular measures during manufacture. A description is given below of how those requirements can nevertheless be fulfilled simultaneously. Despite those measures, however, increasing oxidation cannot be prevented in the case of metal crystallites having a mean diameter of less than 1 nm. In the case of such crystallites, therefore, the oxidation number of less than +2.5 cannot be maintained with the necessary reliability.
The particle size, oxidation number, number of metal and oxygen ligands can be determined on the fresh catalyst by suitable analytical methods. Testing using a transmission electron microscope is suitable for determining the mean particle sizes of the metal crystallites. To that end, the catalyst material is embedded in a suitable composition. A photographic image of a thin section (thickness about 100 nm) of the embedded material is then produced in the transmission electron microscope, and the particle size distribution of the platinum crystallites is determined by evaluation of up to 2000 platinum crystallites.
The mean oxidation number and the mean number of metal and oxygen ligands can be determined by means of X-ray absorption spectroscopy. In particular, the oxidation number is determined by XANES (X-Ray Absorption Near Edge Structure) and the number of metal and oxygen ligands is determined by EXAFS (Extended Absorption Fine Structure). The X-ray absorption spectroscopy is carried out on powder samples compressed to tablets having a thickness of 0.1 mm and with a measuring area diameter of 1 mm, so that it provides a reliable, integral statement regarding the mean oxidation number and the mean number of metal and oxygen ligands of the platinum crystallites over the measured volume.
The zeolites coated with the noble metals are of particular importance for achieving as low a light-off temperature as possible for the hydrocarbons in the diesel exhaust gas. Owing to their acid surface properties, zeolites have a high cracking activity for the long-chained hydrocarbons of the exhaust gas. The long-chained molecules are therefore broken down into smaller fragments by contact with the zeolites, which smaller fragments can then more readily be oxidized by the noble metals deposited on the zeolites.
The catalyst according to the invention does not therefore make use of the storage action of the zeolites for hydrocarbons, as does, for example, U.S. Pat. No. 5,849,255, but uses their catalytic properties to lower the light-off temperature for the oxidation of hydrocarbons as far as possible. Because of the low light-off temperature, large amounts of hydrocarbons are not stored on the zeolites. The proportion of zeolites in the catalyst is therefore calculated not according to the required storage capacity for hydrocarbons but according to the promoting action that they exert on the oxidizing activity of the catalytically active noble metals. As has been seen, that proportion may be kept substantially lower than is the case according to U.S. Pat. No. 5,849,255.
For the catalyst according to the invention, therefore, weight ratios between the support oxides and the zeolites of from 10:1 to 2:1 are totally sufficient, with preference being given to the use of weight ratios of from 8:1 to 3:1 and especially from 8:1 to 4:1. Especially advantageous and ageing-resistant oxidizing activities for carbon monoxide and hydrocarbons have been achieved with a weight ratio of 6:1. A higher proportion of zeolites in the catalytically active coating than corresponds to a weight ratio of 2:1 has an increasingly adverse effect on the catalytic activity. The reason for that is the inhibition of diffusion caused by the relatively small pores of the zeolites, which has a negative effect especially in the case of high space velocities. That effect is lessened by a relatively high content in the catalyst of the mentioned carrier oxides, which generally have substantially larger pores than zeolites and accordingly provide for rapid diffusion of the reactants to the catalytically active metal crystallites.
In order to achieve as high an oxidizing activity as possible for hydrocarbons, a concentration of noble metals on the zeolites that is only from 1 to 50% of the concentration of noble metals on the support oxides is sufficient. Accordingly, taking into account the weight ratio of support oxides to zeolites, there is obtained in the catalyst a mass ratio of the noble metals deposited on the zeolites to the noble metals deposited on the support oxides of from 1:100 to 1:10. The concentration of noble metals, based on the total weight, in the catalyst is preferably from 0.05 to 10 wt. %.
The support oxides aluminum oxide, silicon dioxide, titanium dioxide, zirconium dioxide and aluminum silicate have specific surface areas of more than 5, preferably of more than 50 and especially of more than 100 m2/g. They may be used individually or in the form of a mixture. Binary or ternary mixed oxides of the mentioned support oxides are also suitable, especially an aluminum/silicon/titanium mixed oxide. For stabilization of their specific surface area towards high temperatures, the oxides may be doped in a known manner with suitable stabilizers, such as lanthanum oxide and/or barium oxide. The concentrations of the stabilizing components are, per component, from 0.5 to 20 wt. %, based on the total weight of the stabilized material.
In an especially advantageous embodiment of the catalyst, it contains a mixture of an aluminum silicate and at least one zeolite. Aluminum silicates having a silicon dioxide content of from 0.5 to 20 wt. %, preferably from 1 to 10 wt. %, and a specific surface area of more than 50 m2/g are especially suitable.
Of the large number of zeolites available, the following are especially suitable beta-zeolites, zeolites of the faujasite type, such as, for example, Y-zeolites, especially de-aluminized Y-zeolites, mordenites and zeolites, having a high silicon dioxide content, of the pentasil type, especially ZSM-5. They may be used individually or in the form of a mixture. Such zeolites are preferably used in their acid H+ form. De-aluminized Y-zeolites and ZSM-5 zeolites, each having a modulus of more than 30, preferably of more than 40, have proved especially suitable. The modulus of a zeolite denotes its molar ratio of silicon dioxide to aluminum oxide. Platinum is preferably used as the noble metal.
For the preparation of the catalyst according to the invention, the support materials (support oxides and zeolites) are impregnated with precursor substances of the noble metals based on amine-complexed compounds, such as, for example, ethanolamineplatinum(IV) hexahydroxide, and then calcined. It has been found that amine-complexed compounds are most suitable for maintaining the required particle sizes of from 1 to 6 nm. The formation of small and very homogeneously distributed metal crystallites is assisted by calcination of the impregnated powder material by so-called flash or spray calcination. In the case of spray calcination, the still moist support material impregnated with the precursor substances is blown into a stream of hot gases having a temperature of from 500 to 1000xc2x0 C. and both dried and calcined in the course of a few seconds, generally in the course of less than one second. The powder materials can be appropriately conditioned by the use of gases having a reducing or oxidising action. The optimum dwell times of the powder materials in the hot stream of gas are from 0.1 to 10 seconds. The hot gases required for spray calcination are generally produced by burning an air/fuel mixture, with natural gas preferably being used as the fuel. A description of spray calcination will be found in DE 198 21 144 A1 which is relied on herein for that purpose.
As has been found, spray calcination ensures that the crystallites of the noble metals are distributed very finely on the surface of the support materials, because there is no time during the calcination, which lasts only seconds, for the crystallites to combine to form larger agglomerates.
Loading of the support oxides and of the zeolites with different noble metals can be achieved by preparing the powder materials separately.
The powder materials catalyzed with the noble metals are processed to a preferably aqueous coating dispersion. To that end, they are dispersed in water and ground and homogenized in a ball mill to a uniform particle size of from 2 to 5 xcexcm. They are then applied in the form of a coating to the inside walls of the flow channels of conventional monolithic honeycomb bodies which function as carriers for the catalyst. For fixing of the coating to the honeycomb body, it is dried and calcined. The calcination takes place at temperatures of from 300 to 600xc2x0 C. for a duration of from 0.5 to 4 hours.
In order to fulfil the requirements regarding the oxidation number of the noble metals and the adjacent metal atoms, the coating of the finished catalyst must be reduced in a final operation, for example in a stream of gas containing hydrogen. Forming gas (95 vol. % nitrogen+5 vol. % hydrogen) is preferably used therefor. As combined testing using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XANES, EXAFS) has shown, the noble metals of the catalysts so prepared have an average oxidation number of less than +2 and a number of immediately adjacent noble metal atoms of approximately 4.
The number of oxygen ligands is approximately 2. The test methods TEM, XANES and EXAFS mentioned here are so-called xe2x80x9cbulk testing methodsxe2x80x9d and accordingly provide only average values averaged over the entire catalyst. The individual oxidation number, number of ligands, etc. of a noble metal atom may, of course, deviate from that mean value. These values indicate that the noble metal crystallites have largely been reduced and accordingly are very close to the metal state. That xe2x80x9cmetal-like statexe2x80x9d of the noble metals can be achieved only using noble metal precursor compounds that possess organic ligands, such as, for example, ethanolamine in the case of the compound ethanolamineplatinum(IV) hexahydroxide. By the use of such noble metal precursor substances having organic ligands, autoreduction of the platinum occurs in the course of the calcination, which takes place under an oxygen atmosphere. Platinum particles having oxidation numbers below +2, which correspond to a very high activity, are obtained thereby. A subsequent reduction process in a nitrogen/hydrogen stream leads to a further reduction in the oxidation number and accordingly to an even more pronounced conversion to the xe2x80x9cmetal-like statexe2x80x9d. The coating concentration on the honeycomb body depends on the particular application in question and is generally from 50 to 400 grams per liter of honeycomb body.
The catalyst according to the invention is excellently suitable as an oxidation catalyst for purifying the exhaust gases of a diesel engine, which gases, as a result of measures at the engine, such as, for example, exhaust gas recirculation, exhibit only a very low concentration of nitrogen oxides (less than 100 ppm) and the temperature of which varies in the range from 100 to 500xc2x0 C. during operation.
Tests of the Coking of Support Materials by Adsorption of Hydrocarbons:
The catalyst according to the invention is used in the exhaust gas of diesel engines having predominantly very low exhaust gas temperatures. It is therefore exposed to the risk of contamination and coking by the deposition of VOF""s on the support materials. For that reason, the tendency of support materials to be coked by the absorption and cracking of hydrocarbons has been tested hereinbelow. An aluminum silicate (hereinafter referred to as Al2O3/SiO2) having a silicon dioxide content of 5 wt. % and a specific surface area of 153 m2/g and a Y-zeolite having a modulus of 60 were tested. Both the pure powder materials and powders catalyzed with platinum were tested. The powders catalyzed with platinum were prepared as described in Example 1.
To determine the coking tendency, in each case 2 g of the powder material were placed in a porcelain dish and impregnated with 1 g of a petroleum distillate (mixture of paraffinic, naphthenic and aromatic hydrocarbons in the range from C10 to C16; Hydrosol P 180 HC from Veba-Oel) and then ground in a mortar for 3 minutes. The moist, homogenized powder was distributed on an aluminum film and dried under an infra-red lamp at 150xc2x0 C. for 30 minutes. The cracking activity of the powder so treated was first assessed qualitatively, visually. A quantitative determination of the hydrocarbon deposition was then carried out by oxidation of the hydrocarbon by means of oxygen and determination of the resulting carbon dioxide by UV spectroscopy. The quantitative determination confirmed the qualitative, visual assessment.
The results of that test are given in Table 1. A powder mixture (powder 5 in Table 1) consisting of six parts by weight of powder 2 and one part by weight of powder 4 was also tested.
The results of this series of tests clearly show that the pure support materials without platinum, especially the Y-zeolite, possess a very high degree of cracking activity. On continuous operation of a diesel engine in the low-load range it is therefore to be expected that a large amount of the hydrocarbons contained in the exhaust gas will be deposited on uncatalyzed support materials and cracked. The pores of the support oxides and of the zeolites become xe2x80x9cgummed upxe2x80x9d and are no longer available for the catalytic processes. The result is a marked deterioration in the activity of the catalyst.
If, on the other hand, the support oxides and the zeolites are impregnated with platinum, the tendency to deposition of hydrocarbons falls very considerably, since the fragments of the hydrocarbons formed by the cracking are oxidized on platinum almost completely to CO2 and H2O even at relatively low temperatures of above 150xc2x0 C. The deposition of hydrocarbons and an associated constant deterioration in the catalytic activity are thus prevented. Accordingly, the hydrocarbons that are deposited are continuously burnt. As a result, the accumulation of hydrocarbons on the catalyst is avoided. Such an accumulation would lead at elevated exhaust gas temperatures to a sudden burning of the accumulated hydrocarbons and to the evolution of a large amount of heat, and would damage the catalyst thermally.